For further enhancement of vehicle safety, it has been an increasing strong demand to improve the braking performance and the steering stability of tires contacting with a road surface. Another increasing strong demand is to lighten tires for improved fuel efficiency in terms of environment preservation. To this end, there has been proposed a radial tire described in Japanese unexamined, published patent application No. 62-152904. In the known radial tire, body ply cords which extend from a tread section to each sidewall section to be turned up over a bead core of a bead section are arranged in a radial cord fashion (arrangement at an angle in the range of 85 to 90 degrees relative to the tire circumferential direction), a secondary belt layer in which steel cords are inclined at an angle in the range of 20 to 40 degrees relative to the tire circumferential direction is arranged as a single layer over an outer circumference for a tread section of the body ply in the tire circumferential direction, and a steel cord or an organic fiber cord such as Kevlar® which is large in the increase rate of tensile load to stretch rate is spirally wound on an outer circumference side of the oblique belt layer almost in parallel with the tire circumferential direction to form a main belt layer. The belt layers are to prevent the movement of the body ply by a so-called “hoop effect” which suppress the radial expansion of the body ply for securing the steering stability, high-speed endurance and wear resistance which are characteristic performances of radial tires. As belt cords arranged in the belt layers, there have been used steel cords and aromatic polyamide fibers which are large in the increase rate of tensile road to stretch rate for a greater “hoop effect”.
On the other hand, in Japanese unexamined, published patent application No. 8-318706, there is described a radial tire wherein PET (polyethylene terephthalate) fiber or nylon fiber which is small in the increase rate of tensile load to stretch rate is used for a circumferential belt layer with the ply structure appropriated.
Further, for a proper “hoop effect” by belt layers, a trial manufacture has been made of a radial tire, in which as shown in FIGS. 19 and 20, two secondary belt layers 71 and 72 whose respective steel cords are inclined in mutually opposite directions relative to the tire circumferential direction are arranged on an outer circumference side for a tread section of a body ply 70 in the tire circumferential direction, and an organic fiber cord is spirally wound on an outer circumference side of the oblique belt layer 72 at the outside almost in parallel with the tire circumferential direction thereby to form a main belt layer 73.
In vulcanizing a so-called “green tire” formed with unvulcanized compounded rubber in a manufacturing process of tires, the green tire is put in a mold and is vulcanized and formed in the mold with heat and pressure being applied inside. To obtain a target tire shape by the vulcanization and forming, the green tire is expanded within the mold to add a stretch of several percents or, in dependence upon the type of the forming method, 0.5 to 3%. Where there are employed conventional steel cords or aromatic polyamide fibers which hardly stretch because being large in the increase rate of tensile load to stretch rate, a problem arises in that they cannot be stretched by the pressure applied to the green tire up to 0.5 to 3%, whereby the green tire cannot be formed to a correct shape as a result that the body ply cords are prevented by the non-stretchable belt cord from being expanded. On the other hand, in the case of a radial tire which employs as belt cords PET or nylon fiber cords which are small in the increase rate of tensile load to stretch rate, there arises a problem that a sufficient hoop effect cannot be obtained after the vulcanization and forming.
The present invention is made to solve the aforementioned problems and is to provide a radial tire with a belt layer in which there is circumferentially spirally wound a belt cord capable of allowing the green tire to expand with a correct shape retained during a vulcanizing and forming process and also capable of exercising a sufficient hoop effect after the vulcanizing and forming process.